Josephson Junction Description (JOS)

Note that there is NO eating or drinking in the 111-Lab anywhere, except in rooms 282 & 286 LeConte on the bench with the BLUE stripe around it. Thank You the Staff.

Superconducting phenomena are among the most fascinating features in physics, and the Josephson effect is the most interesting of all. In 1962, Brian Josephson predicted that electron pairs could tunnel without resistance through an insulating barrier between two superconductors. Here we use a point contact between niobium wire with a thin oxide layer and a niobium block. Both the DC and AC Josephson effects are observed and measured. A DC current can flow through the junction with no potential difference, but when a DC voltage is applied together with a small alternating voltage, the I-V curve shows a characteristic step structure. From this step structure the value of 2e/h can be calculated easily and accurately, an exciting consequence of the properties of the junction. The ratio e/h appears throughout atomic and condensed matter physics.

In this experiment you will learn some low temperature techniques which are used in many research laboratories. You will also learn some practical and often frustrating electronics, together with manual skills in making niobium points and adjusting them to produce a useful electronic signal.

Pre-requisites: Physics 137A (may be taken concurrently)

Days Alloted for the Experiment: 8

Consecutive days: Yes

All pages in this lab. Note To print Full Lab Write-up click on each link below and print separately

I. Josephson Junction

This lab will be graded
40% on theory,
40% on technique, and
20% on analysis. For more information, see the
Advanced Lab Syllabus.

Before the Lab

Complete the JOS Pre Lab and Evaluation sheets. Print and fill it out. The Pre-Lab must be printed separately. Discuss the experiment and pre-lab questions with any faculty member or GSI and get it signed off by that faculty member or GSI. Turn in the signed pre-lab sheet with your lab report.

You should keep a laboratory notebook. The notebook should contain a detailed record of everything that was done and how/why it was done, as well as all of the data and analysis, also with plenty of how/why entries. This will aid you when you write your report.

Intent and scope of this laboratory manual

This manual is intended to provide general guidance for the lab. It is not designed to explain all the physics necessary for understanding this experiment. Nor is it a "cook-book" telling you exactly which buttons to press, etc. It is up to the student to become familiar with the necessary material in the reprints, and to figure out the exact procedure necessary for completion of the assignments in the lab. Talk with an instructor often, but not until you have given the physics and procedures some thought first. We are here to instruct and help, but not to do your thinking for you.

Introduction

In this experiment you will construct a Josephson junction, use it to observe the DC and AC Josephson effects, and measure the ratio of fundamental constants

Apparatus

The junction

The central component of this experiment is the point-contact junction (PCJ), shown in Figure 1. It is a niobium needle that is delicately pressed against a niobium screw. Both pieces have oxide layers that form a barrier to electrical conduction between the niobium pieces. The thin oxide layers and the small contact area between the needle's point and the screw provide the weak-link necessary to observe Josephson tunneling.

Figure 1: The Junction

The junction assembly

The junction is mounted in an assembly shown in Figure 2. To adjust the pressure in the junction, we use a mechanism called a differential screw (D), which moves the niobium needle point towards the niobium base screw (FB) with a great deal of control. The needle (A) fits into a hole in a form (B) made of a kind of fiberglass called "G10." The needle is held in place by two screws (not shown in figure 2 \[see figure 9\]). The body piece (B) can slide up and down on the two stainless steel rods (C). The flat base (FB) of the junction is held rigidly in place in G10 and cannot slide up or down. The differential screw (D) has pitch of 72 threads per inch at the top and a pitch of 80 threads per inch at the tip fixed in (B).

When the differential screw (D) is advanced one turn, it moves down 1/72". But part (B), which cannot rotate, moves up 1/80". The net motion of the needle with respect to the Nb base is thus only 1/720" (= 1/72" - 1/80"). Even with this effective pitch of 720 turns per inch, the screw must be turned very slowly to prevent damaging the needle point.

Figure 2: Junction Assembly

The Cryogenic Probe assembly

The junction assembly is mounted in a probe, manufactured from a ~three foot length of 1/2" diameter Stainless Steel Tubing (SST), as shown in Figure 3. This tube serves as a holder for the junction assembly and as a waveguide for the microwaves used in the observation of the AC effect.

The junction end of the probe is inserted into a cryostat filled with liquid helium. The other end remains outside the cryostat and provides the electrical connections (not shown), radio-frequency (RF) coupling port, and differential screw manipulator (labeled Hand adjustment). The junction is held in the probe by set screws (F). When the screws are turned counterclockwise (advancing them outward), the screw heads engage in a retaining hole in the SST wall, locking the assembly in place. (Note that as the screw holes in the assembly were not drilled 180 degrees apart, while the retaining holes were, both set screws cannot be engaged simultaneously. One screw goes through a retaining hole, the other can be adjusted to keep the fit secure.)

Figure 3: The Probe

I-V Measurements

All of your measurements will be made by examining the current-voltage (I-V) characteristic of the junction. The I-V curve is traced by measuring the voltage that develops across the junction as the current is swept by a low frequency (~60 Hz) oscillator. Since the impedance of the junction is small, a standard four-terminal measurement must be used. Four Wire Measurement .
This is a very important way of measurement for this experiment. You will get a resistive measurement instead of a I/V curve measurement.

The electronic hookup is shown in Figure 4. The parts in the dashed area are physically located in the same small gray box with a clear plastic cover. The four electrical connections to the junction are labeled P1, P2, and B1, B2.

Figure 4: Electronics Block Diagram

The Radio-Frequency electromagnetic radiation (RF)

The RF is generated by a Hewlett-Packard model 8350B sweep oscillator running in continuous-wave (CW) mode, outputting maximum power of ~20 dBm. The oscillator output is connected to an active doubler, a semiconductor device which amplifies the output by ~ 20 dB (up to a maximum of ~ 29 dBm) and doubles the frequency. For example, setting the HP to 11 GHz and 5 dBm will result in ~25 dBm of 22 GHz RF out of the doubler. The doubler is mounted to a heat sink and to the wave guide. The wave guide connects through an isolator, a power splitter (directional coupler), an attenuator, and finally to the cryogenic probe. The isolator prevents potentially damaging reflections from returning to the doubler. The power splitter picks off a fraction of the RF power and sends it to an RF frequency measurement instrument consisting of a variably-tuned absorption cavity and a crystal RF power detector. The output of the crystal detector is an electrical signal that is proportional to the RF power incident on the detector. RF frequency measurement is achieved by tuning the absorption cavity until a reduction in the magnitude of the detector output is observed. The frequency is then read off the graduated scale on the absorption cavity. Its accuracy is much better than that of the RF source.

Figure 5: The RF Equipment

Cryogenics Safety

The liquid helium is contained in a ~ 5 foot tall insulated vessel called a cryostat. You must notify the lab staff at least one day in advance that you will need liquid helium for your lab and before it is near empty (about 4 liters remaining).

Procedure

Prep and Clean-up

There are many little parts and tools associated with the apparatus. Make an effort to keep the area clean. When you leave for the day, assemble the probe and put it back in its holder, get the cables off of the floor, and collect the tools.

Making a point

Take a short Nb wire (around 5/8 of an inch long) and shape it with a file. It is best to use the pen vice at your station to hold the point. File evenly around the point by rotating the vise so you get a conical shape, with sides at roughly 45 degrees to the axis of the point. (Significantly smaller angles will weaken the point, while significantly larger angles may make for a messy contact.) After shaping it with the file, smooth the surface of the tip with emery paper, or fine sandpaper. (There should be a variety of grades of sand paper under the table.) Check the appearance of the point by eye or with a microscope. The appearance should be something like Figure 7. Holding the point over a white surface, like a sheet of paper, can make it easier to see its shape.

Figure 7: Nb Tip Magnification

By far the best results are obtained by leaving the point out in air to oxidize overnight. You can get a working point right away by using a hot plate, but it is extremely difficult to get the desired amount of oxidation and avoid introducing impurities. If you absolutely need a working point right away, preheat the hot plate. Then put your point on the plate for approximately 25 seconds to a minute, or until the point achieves a nice brassy color. Don't have high expectations for this hot plate point, though.

It is recommended that you spend the first day of this experiment making the points that you will use in the succeeding days. You should make at least five right away. Once you've done this, take some time to familiarize yourself with the equipment; you don't have to take data your first day.

Remove the junction assembly from the probe

This is the most delicate part of the lab. Please work over a clean surface, exercise caution, use your good judgment, and consult the lab staff if you have any uncertainty about what to do. Please refer to Figure 2, repeated here for your reference, as well as photographs of the junction assembly in Figure 8.

Figure 2 (again): Junction Assembly

Two tiny set screws (F), which are not on opposite ends of the diameter, hold the junction in the probe. To remove the junction, one of the set screws must be turned in (clockwise) to the fiberglass housing, so that it will clear the holes in the stainless steel tubing. Using an allen head wrench, gently turn the set screw clockwise until it clears. Check to see that the wire leads are free at the upper end of the probe, (i.e., the rubber cork should be out). Then gently pull the junction out of the tube. A slight twisting action helps.

When reinstalling the assembly, the crucial point is that there should be no resistance to backing out the set screw. The hole must be lined up exactly and the set screw backed out only far enough to hold the junction in place. No attempt should be made to tighten it.

Installing a point in the assembly

There are many tiny parts (screws, etc.) in this assembly that can be easily lost or damaged. When disassembling the apparatus, please do it directly over a clutter-free bench top, so that when the little parts do fall, they don't make it to floor to roll around and get lost. Get a container in which to put the parts. Take all the disassembled parts and store them in a safe spot where you will be able to find them when you are ready to reassemble the apparatus. Use a minimum of force and don't be afraid to ask for help if anything seems stuck.

To install a new point, you will probably have to first remove an existing one: First back off the differential screw (D) until it comes out of the housing. Loosen the nuts associated with the screws (See Figures 8 and 9) which have solder lugs attached to them, in piece (B). You usually don't need to remove the nuts entirely (and its easier if you don't). Using small needle-nose pliers, gently pull the needle out of the housing (B). Do this over a clean surface because the screws will probably fall out as you remove the needle. Throw the old needle away.

Before you install a fresh point, use emery paper (or light sand paper) to remove the oxide layer from the back end of the point (the last 3/8" or so), where it will meet the electrical contacts. Be careful you don't disturb the oxide layer on the tip of the point. Thread the new point through the lower screw shaft, making sure the solder lug corresponding to P1 is properly attached (see figure 9; note that the wires are color coded). Then thread the point through the upper screw shaft, with the lug for P2 properly secured. Tighten the nuts tight enough to make good electrical and mechanical contact, but not too tight. Too much tightening will bend the needle making it nearly impossible to remove. While holding the point close to the Nb base, but not touching it, replace the differential screw. Be careful that the tip of the differential screw does not touch the back end of the niobium needle, inadvertently grounding it. When the junction assembly is inserted into the tube, the screws and solder lugs may come in contact with the tube's inner wall, shorting them; it may be desirable to place insulating material, such as small strips of electrical tape, over these exposed connections (don't use very much; in the tube, space is limited).

Replace the junction assembly in the probe tube. As you do this, make sure that the socket in the top of the differential screw mates securely with the drive shaft running the length of the probe. It also helps to have your partner gently take up the slack that develops in the wires as you move the assembly into the probe. Secure the assembly by backing out the retaining set-screw.

After you get everything hooked up, be careful with the probe; lay it flat on the table when its not in the cryostat (you don't want to loose all your hard work to a drop or a kick).

Figure 8: The Junction Assembly

Check for proper electrical continuity

After installing the junction in the probe, check to see that an infinite resistance separates each output (B1, B2, P1, and P2) from ground (the metal body of the probe tube). B1-B2 as well as P1-P2 should show continuity. With an open junction, P1-B1, P2-B1, P1-B2, P2-B2 should show infinite resistance. You should do these checks before every immersion in liquid helium.

Figure 9: Details of Point Holder Construction

Make contact between the point and the base

Connect an analog Ohm meter (e.g., a Simpson VOM) between P1 and B1. Set it to a scale that would display 50 Ohms easily. It should show an open circuit. Slowly tighten the screw at the top of the probe. Don't over-tighten the screw or you will scrape off your oxide layer and have to start all over. After a few turns, the VOM ought to suddenly show a small resistance, approximately a few Ohms, or tens of Ohms. If you can't get this behavior your point may need to be replaced.

Connect the probe to the I-V curve electronics

Connect the probe to the electronics box, current oscillator, preamp, and scope (look back at Figure 4). Good initial settings for the electronics are...

Scope: Set the scope for X-Y mode with 1 V/division for both axes and DC coupling.

You should see a straight line on the scope with the preamp turned off. You may have to play with a number of the above settings; every point is different. The Low-Frequency oscillator is set to 60 Hz to match any 60 cycle pick up, but the wall current may vary (20 mHz, so you may need to adjust the frequency slightly to get a signal on the scope that doesn't oscillate. By using the slope of the I-V line, calculate the room-temperature resistance of the junction. It should be the same as what you measured on the VOM.

There is a LabView program to collect the data from the digital scope.

Before you Insert the probe into the cryostat

Liquid helium is expensive. So you must be careful in order not to waste it. As mentioned, each time you go to cool the probe, you should check for shorts, proper continuity, and proper junction resistance. Only if everything checks out and is working properly, may you proceed to insert the probe into the cryostat. You should only need to immerse each point once. As most of the helium boil-off occurs with the cool down, it is the number of immersions, not the length of time the sample is immersed, that will determine how much LHe is used.

Liquid helium has a low heat capacity. So you must be gentle when handling the cryostat. You can boil off a lot of extra helium just by shaking the cryostat or making excess turbulence inside it. To avoid excessive boil off during insertion, it is useful to know how much LHe is in the cryostat. Open the valve at the top of the cryostat. It will vent for a few seconds. There should be a long, thin (< 1 cm) metal tube with a latex membrane over one end located at your station. Hold a finger over the membrane and lower the tube into the cryostat; note when it touches the bottom. As LHe boils off, He vapor traveling up the tube should cause the membrane to vibrate. Lift the tube and note its height when you feel the vibration change; this gives you the depth of the LHe.

Before you cool the probe, check that it is completely dry; any leftover condensation from previous cooling cycles on the exterior of the probe could quickly form dangerous, neck clogging ice, while and liquid in the point assembly could ruin the experiment. Remove all the electrical connections from the probe, and check that the rubber plug (R) and RF coupling port cap are securely in place.

Carefully insert the probe into the cryostat, turning it back and forth as you go to prevent sticking. When the probe is an inch above the surface to the LHe, as you measured it, slow up its descent. Helium will rush out past the sides of the probe as you immerse it. Don't go too fast. The sound of the rushing helium should be soft and quiet, if it gets too loud (if you can hear it above the din in the 111-lab, it probably is) stop lowering the probe until it subsides, but don't stop turning it. The probe is more likely to freeze to the cryostat during excessive outflow. If the probe freezes into the storage vessel neck such that it cannot be turned or removed, call a staff member immediately. This is potentially quite dangerous as pressure in the vessel could build up.

Every day, before you go home, you should remove the probe from the cryostat; if the probe is left unattended overnight, the neck could freeze closed and there could be an explosion. You should make sure you close the cryostat valve after the probe is removed to avoid wasting LHe. Whenever the probe is taken out of the liquid helium, it should be dried. There is a heat gun on the lab table; use it to remove any condensation and heat the entire probe, including the point assembly, until it is warm to the touch (not hot: you don't want to melt anything). Even if you are leaving the probe overnight, it is important to dry it, as you some condensed water may not evaporate.

Observe the DC effect

Figure 10: I-V curve of DC effect

Connect the probe outputs to the electronics and observe the I-V curve displayed on the scope. Gently turn the screw to obtain the DC effect. You may very quickly see an I-V curve that looks like the DC effect (see Figure 10). Test whether this is a true DC Josephson effect or a supershort by bringing the large permanent magnet near the cryostat. If you have the DC effect, you should see fluctuations in the maximum current that flows without a voltage drop as you move the magnet around. If you do not observe these fluctuations, keep adjusting. You should find that the DC Effect is not too difficult to get, although it takes patience. If you don't get a good effect, try subtly raising and lowering the point with the screw; each time you do this, you make the contact at a slightly different location (were talking Angstroms here) with potentially different properties at the same time, with each contact, some of the oxide layer is removed (which may or may not be a good thing). When you have a good DC effect, download yourr data from the scope with the [JOS Program]

First you should use the analog scope to see all of your data DC and AC effects.

Observe the AC effect

Once you have a good DC effect, you are ready to get the AC effect by applying the microwaves. First turn on the RF Oscillator (Figure 11) and the active doubler. Then measure the RF power. Set the attenuator to 20 dB, then attach the power meter to the waveguide. Set the power meter's scale to 10 mW. You must keep the attenuation at a value that gives a power reading without overloading the meter. The thermistor detector, connected to the waveguide, may be damaged if you apply it more than 15 mW power (15mW maximum), so be careful. That is why the microwave attenuator is set to 20dB to start.

Set the RF source to a frequency in the 9 - 13 GHz range( this is sent to the frequency doubler to get 18-26 Ghz) by pressing the "CW" button followed by the freq on the numeric keypad (e.g., press CW, 1, 1, GHz for 11 GHz). The display should look like the picture below (Figure 11). You can adjust the frequency with the knob below the display, and it's a good idea to do so when you search for the AC effect.

Figure 11: RF Oscillator

Practice measuring the frequency. Tune the absorption cavity in the neighborhood of twice the displayed frequency of the RF source. When you observe a decrease in magnitude of the detector output, the cavity is tuned to the RF frequency, which may be read off of the cavity's scale.

The RF system will generate about 50 to 350 mW of power (The power can be more and that's okay). Record how much power you're getting (based on the power meter readings and the attenuation, as well as the frequency. Then connect the waveguide to the probe.

To find the AC effect, you really only have three variables at your disposal: RF power, RF frequency, and junction pressure. To vary the power, either use the attenuator, or the power output knob on the RF source (the former is probably best). Tweak the pressure of the point contact as you watch for faint steps. Most probably you will need to vary the microwave power and the pre-amp / scope settings to see the effect. The best advice is to play with everything at your disposal to obtain the effect. When you have as many clear steps as you can find, take a photo of the scope display for analysis. The AC effect is a little more difficult to achieve, but everyone does get it after some adjustments; have patience and be gentle. Once you have the AC effect, leave the RF frequency set-up undisturbed; you will want to make a precise frequency measurement after you have calibrated the PRE-AMP and scope.

First you should use the analog scope to see all of your data DC and AC effects.

Then use the Josephson Junction Interface Program to see and transfer your data to the computer.
[Josephson_Program]

Figure 12: I-V curve of AC effect

Calibration

As you might imagine, the accuracy of your results depends critically on the calibration of the voltage axis of the I-V curve. One method of calibration would be to measure the gain of each individual piece of the signal chain (junction to pre-amp to scope) and multiply them together. But that becomes impractical as the number of signal chain elements grows, and is more prone to error than the alternative. So, although the signal chain in this experiment is not too complex, you are going to use the technique of measuring end-to-end gain. Replace the Josephson junction, two wire cables, with a voltage box about 0.513mV. Measure the box voltage with the DVM and hook it up to replace the point contact voltage. Measure the limits of the excursion of the trace on the scope and use this to calibrate your equipment setup.

Specifically:

Determine the number of divisions that the dot spans on the scope.

Using the digital volt-meter (DVM)at the lab setup, measure the RMS amplitude of the signal that you are applying to the pre-amp.

Be sure to record estimates of your measurement uncertainties. Calculate the end to end gain.

Make a precise measurement of the RF frequency using the absorption cavity and make an estimate of 2e/h. What do you trust more the synthesizer reading or the cavity measurement? As with all experiments, you should be calculating and checking results in lab, as you go through the experiment. Don't take data and expect that it will all work out later.

Report

Be sure that your report includes the oscilloscope pictures with the axes clearly labeled and scaled for quick identification.

Compare your value for 2e/h with that obtained by Parker, Taylor and Langenberg The Josephson Effects (Phys. Rev. Letters 18, 287 (1967)) of (483.5912 + 0.0012) MHz/microvolt. Explain any major discrepancies between your results and theirs. What are the errors on your measurements? Are your errors consistent with the quality of your data?

When displaying the I-V characteristic, which of the axes needs high band-width. Why do we not need high band-with for the other axis?

What physical significance does an accurate measurement of the ratio e/h have?